In the oil exploration industry, seismic data is obtained to enable scientists and engineers to develop a picture of underlying rock formations. The reflection seismic method attempts to image the top few kilometres of the earth's crust by artificially creating a wavefield at the earth's surface and then recording this wavefield at multiple locations as it returns to the surface via reflections from the rock layers of the earth's crust. These wavefields are then processed in order to obtain images of the subsurface that can be used to help locate hydrocarbons or other minerals. In order to obtain this data, a wavefield is created at the surface at a source location by setting off a percussive shock wave that imparts wave energy into the ground. The source is typically an explosive charge, Vibrator sinusoidal wave or a mechanical impulse system. A Vibrator creates a sinusoidal signal of changing frequency through shaking the earth, whereas an impulse or explosive source creates a single multiple frequency shock wave that travels into the earth.
A series of receivers (geophones) located at previously surveyed points are set up to record the amplitude of wave energy reflected to each receiver point from underlying formations as a function of time, thus creating an array of time/amplitude data sets from each geophone array.
As noted, shock waves can be imparted to the ground by either explosive or mechanical systems. While explosive systems can generate shock waves of a greater magnitude, there are many disadvantages in using explosives both in terms of regulations and efficiency. Thus, mechanical impulse systems are desirable due to: low deployment cost, high resolution data created by a greater number of source locations, increased safety and low environmental impact.
In the past, however, mechanical shock wave generators have been disadvantaged in that the amount of impulse energy imparted to the ground cannot be accurately controlled thus leading to increased error margins in the interpretation of the collected seismic data. Thus, there has been a need for a mechanical impulse system that is capable of precisely controlling the amount of impulse energy for a given source location.
A review of the prior art reveals that a system that enables a precise amount of energy to be delivered to the ground has not yet been deployed.
For example, U.S. Pat. No. 4,271,923, U.S. Pat. No. 4,402,381 U.S. Pat. No. 3,905,446, U.S. Pat. No. 4,118,994 and U.S. Pat. 4,316,521 describe hammer/anvil pulse generator systems. However, none of these patents describe the use of a hydraulic actuation and control systems that provides linear velocity and position feedback to ensure that a repeatable output of energy is delivered to the earth.
U.S. Pat. Nos. 4,341,282, 4,011,923, 4,114,722, 4,135,598, 4,116,300, 5,666,328, 6,065,562 and U.S. Pat. No. 4,492,285 each describe pulse generator that utilizing a vibrating energy source, U.S. Pat. No. 4,108,271 describes a pulse generator that releases pressurize gas to impart energy to the ground that does not utilize a hydraulic actuation and control system that provides linear velocity and position feedback to ensure that a repeatable output of energy is delivered to the earth. U.S. Pat. No. 3,557,900 describes a pulse generator that utilizes a chemical combustion process.